1. Field of the Invention
The present invention relates to an optical amplifier having an optical fiber and, more particularly, to a gain-controllable optical fiber amplifier and method of controlling the amplifier pump power.
2. Technical Background
Data communication systems increasingly are employing optical fibers as the transmission paths for information. The use of optical fiber generally allows for the transmission of large amounts of data at high speeds for long distance transmission. Optical fiber, although to a lesser extent than other transmission mediums such as wire, usually is susceptible to some signal loss such that signals transmitted therewith are attenuated as the transmission path becomes longer. As a consequence, one or more optical amplifiers are often employed in the communication path to amplify the optical signals.
Various types of optical amplifiers have been developed and are used to amplify optical signals. In particular, the erbium-doped fiber amplifier (EDFA) is one example of an optical fiber amplifier that is widely known for use in amplifying optical signals. The erbium-doped fiber amplifier is a rare earth element-doped optical fiber amplifier containing erbium, as the rare earth substance, injected into the optical fiber and raised to an excited state by pump light that is input separately from the signal light so that the signal light is amplified by the pump energy. Accordingly, the amplifier generally requires one or more pump light sources coupled to the optical fiber.
In recent years, the amount of information transmitted on optical fibers has increased significantly. Typically, to increase the capacity of the optical transmission path, several light signals at different wavelengths (i.e., channels) are multiplexed for transmission through a single optical transmission. In a multiple-wavelength multiplexed transmission system, a plurality of channels are made available for transmission by an optical fiber. However, the total number of active channels that are used may fluctuate as channels are added or dropped depending on the demand, or in the event that channels fail. In order to maintain a constant per-channel gain, the gain of a fiber amplifier used in the systems generally must be controlled in response to changes in the number of active channels. Accordingly, the greater the number of channels that are multiplexed on an optical fiber, generally the greater the amount of pump energy that becomes necessary to maintain a constant per-channel gain, and vice versa.
While the source of pump light may be controlled in conventional optical amplifiers to provide a substantially constant overall gain, a number of drawbacks exist. Conventional constant gain amplifiers do not completely compensate for changes in the number of channels. In conventional amplifiers, changes in the number of channels generally leads to noise figure degradation and per channel power variations. In particular, multiple-pump rare earth element-doped fiber amplifiers commonly have an internal element, such as a dispersion compensation filter, that is generally sensitive to high per-channel signal powers. Gain or power control achieved by varying the pump power of the rare earth element-doped fiber may present noise figure degradation, especially when the total input power becomes comparable to backward amplified spontaneous emission (ASE) at the input of the amplifier coil. For this reason and others, it is desirable to provide a gain controllable fiber amplifier that minimizes the amount of noise introduced by the amplifier.